The present invention relates generally to the detection and measurement of forces and more particularly to an improved accelerometer incorporating one or more vibrating force transducers for measuring the force applied to a proof mass. The present invention also relates to a method for manufacturing the accelerometer.
A widely used technique for force detection and measurement employs a mechanical resonator having a frequency of vibration proportional to the force applied. In one such mechanical resonator, one or more elongate beams are coupled between an instrument frame and a proof mass suspended by a flexure. An electrostatic, electromagnetic or piezoelectric force is applied to the beams to cause them to vibrate transversely at a resonant frequency. The mechanical resonator is designed so that force applied to the proof mass along a fixed axis will cause tension or compression of the beams, which varies the frequency of the vibrating beams. The force applied to the proof mass is quantified by measuring the change in vibration frequency of the beams.
Recently, vibratory force transducers have been fabricated from a body of semiconductor material, such as silicon, by micromachining techniques. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical resonator device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. Nos. 5,006,487, "Method of Making an Electrostatic Silicon Accelerometer" and 4,945,765 "Silicon Micromachined Accelerometer", the complete disclosures of which are incorporated herein by reference.
The present invention is particularly concerned with accelerometers having vibrating beams driven by electrostatic forces. In one method of fabricating such miniature accelerometers, a thin layer of silicon, on the order of about 20 micrometers thick, is epitaxially grown on a planar surface of a silicon substrate. The epitaxial layer is etched, preferably by reactive ion etching in a suitable plasma, to form the vibrating components of one or more vibratory force transducers (i.e., vibrating beams and electrodes). The opposite surface of the substrate is etched to form a proof mass suspended from a stationary frame by one or more flexure hinge(s). While the opposite surface of the substrate is being etched, the epitaxial layer is typically held at an electric potential to prevent undesirable etching of the epitaxial layer. During operation of the transducer, the beams and the electrodes are electrically isolated from the substrate by back biasing a diode junction between the epitaxial layer and the substrate. The transducer may then be coupled to a suitable electrical circuit to provide the electrical signals required for operation. In silicon vibrating beam accelerometers, for example, the beams are capacitively coupled to the electrodes, and then both the beams and electrodes are connected to an oscillator circuit.
The above described method of manufacturing force detection devices suffers from a number of drawbacks. One such drawback is that the beams and electrodes of the vibratory force transducer(s) are often not sufficiently electrically isolated from the underlying substrate. At high operating temperatures, for example, electric charge or current may leak across the diode junction between the substrate and the epitaxial layer, thereby degrading the performance of the transducer(s). Another drawback with this method is that it is difficult to etch the substrate without etching the epitaxial layer (even when the epitaxial layer is held at an electric potential). This undesirable etching of the epitaxial layer may reduce the accuracy of the transducer.
Another drawback with many existing force detection devices, such as accelerometers, is that they often have an asymmetrical design, which may make it more difficult to incorporate the accelerometer into a system, particularly in high performance applications. For example, the proof mass flexure hinge is typically etched on the opposite surface of the substrate as the transducers. This produces an asymmetrical device in which the input axis of the accelerometer is skewed relative to a direction normal to the surface of the silicon wafer.